The municipality of Castrillo de Villavega (202 inhabitants in 2009;
3,394 ha; unofficial website) is located 60 km from Palencia. The village is made of two parts, Castrillo, the biggest one, and Villavega, separated by river Valdavia, on which a wonderful stone bridge was built in the 19th century, eventually linking the two parts of the village.

Castrillo de Villavega appeared in the history as Castrillo de la
Villa Vega. The castle of Castrillo de la Vega, today ruined, was
probably built in the 10th century during the early resettlement of
the area, as an outpost watching the border with the Muslim states.
The castle is made of a rectangular tower (6.30 m x 6.70 m) protected
by a circular wall.
In 1204, King Alfonso VIII bequeathed in his testament the citadel of
Castellum de Villalega to the Order of St. John of Jerusalem. On 26
April 1279, King Alfonso IX the Wise granted the domain of Castrillo
de Villavega to Johana Gómez, the wife of Infant Luis, himself the son
of King Ferdinand the Saint; she was succeeded on 21 July 1305 by
Mencía de Manzanedo and, later, by her son Juan Rodríguez de Cisneros.
Cisneros was a "rich man" (noble of high rank), who fought in 1367 in
the Battle of Nájera, together with King Henry II; he died without a
male heir in 1370. In 1418, Castrillo de Villavega was transferred to
Leonor de la Vega, heiress of the Vega lineage after the death of her
father Garcilaso de la Vega in the Battle of Nájera. Leonor was
succeeded in 1456 by her daughter Mencía Sandoval de la Vega, who
bequeathed on 21 August 1514 Castrillo de la Vega to the Duke of
the Infantado. The village would be ruled "quietly and pacifically" by the
Dukes until the suppression of the feudal system in 1837; Pedro
Alcántara Toledo Salm Salm, 12th Duke of the Infantado, was the last lord of Castrillo de Villavega.

The flag and arms of Castrillo de Villavega are prescribed by a Decree
adopted on 20 December 2001 by the Municipal Council, signed on 8 April 2002 by the Mayor, and published on 7 May 2002 in the official
gazette of Castilla y León, No. 85, p. 5,952 (text).
The symbols are described as follows:

Flag: Rectangular, with proportions 2,3, made of three
horizontal stripes with proportions 1:2, 1:4 and 1:4, the upper stripe
red with a white Cross of St. John of Jerusalem placed at 1/3 from the
hoist, the central stripe white and the lower stripe blue.Coat of arms: Gules a castle argent masoned port and windows sable
ensigned with a Cross of St. John of Jerusalem argent in base waves
argent and azure. The shield surmounted with a Royal crown closed.